This invention relates to an electrical disconnect system for protecting transient voltage surge suppressor components and the surge protected utilization equipment connected thereto from power frequency overvoltages.
Metal-oxide-varistors (MOVs) are the most widely used surge-suppressor components employed in devices for suppressing transient voltage surges in low voltage (120/240 volt) industrial, commercial and household electrical distribution systems. Their low cost and effectiveness have greatly contributed to their widespread use. Metal-oxide-varistors are subject to failure from transient voltage surges that exceed their maximum energy absorption capability or their pulse lifetime ratings. It is believed by many observers, however, that most of the failures of these devices that occur in typical low voltage electrical distribution applications are not related to the MOV's transient voltage surge exposure but are caused by power frequency overvoltages. The manner in which these devices fail from power frequency overvoltages is often violent and dangerous. Such failure modes can result in severe overheating and a subsequent cause of fire and/or electric shock hazard. The safety hazards resulting from MOV failures have become recognized to the extent that the Underwriters Laboratory Standard for Transient Voltage Surge Suppressors (UL 1449) has recently been revised to deal specifically with the question of safe failure modes for metal-oxide-varistors when subjected to power frequency overvoltages.
To assure safe failure modes from various causes of MOV failures the surge suppression devices must be subjected to electrical stresses that induce such failures. Stress tests have been devised that simulate overvoltage conditions caused by the electrical distribution problems outlined below that are known to cause such failures:
1. Power frequency overvoltages with high current available. Such overvoltages can occur from voltage "swells" incident to utility operations such as power grid automatic switching during severe storms to maintain service when part of the system has been damaged. Such power frequency overvoltages can cause overheating of varistors resulting in "thermal runaway" and violent failure modes, which can rupture the MOV body and cause the emission of flames or molten metal.
2. Wire-crosses caused by electrical system damage or accidents. Such accidents or damage to the distribution system can result in the impression of extreme overvoltages such as twice or more the normal line voltage with high available current levels. Such overvoltages usually result in explosive violent failure modes.
3. Limited current power frequency overvoltage. Such a condition can result from a defect or accident in the electrical distribution system. The most common cause is a "dropped" or open neutral conductor in a three-phase "wye" connected system or split-single-phase 120/240-volt system (such as a typical household electrical supply). The overvoltage causes an extreme temperature rise in the MOV even though the current may be too small to activate any upstream overcurrent protection device such as a fuse or circuit breaker. This is one of the most dangerous failure modes because the overheated MOV can ignite nearby flammable materials and result in an "electrically fed" fire.
To test for safe failure modes under the conditions described above, UL has included a series of "Abnormal Overvoltage Tests" in the new UL 1449 Standard:
Temporary overvoltage (TOV)--high current abnormal overvoltage test. This is a seven-hour test at 125% of rated supply voltage from a 1000 ampacity source. PA0 Full phase voltage--high current abnormal overvoltage test. This test applies twice the maximum rated supply voltage (240 volts) with high current (1000 amperes) available. PA0 Limited current abnormal overvoltage test. This test applies twice-rated normal operating voltage but limits the current to a value below the trip point of any over-current protection devices in the supply line. The test results in non-violent intense heating of the MOVs.
FIG. 1 is a schematic diagram of a prior art surge suppressor employing three metal-oxide-varistors MOV1, MOV2 and MOV3. It is designed to provide protection from transient voltage surges that appear across any pair of conductors PHASE to NEUTRAL, PHASE to GROUND, and NEUTRAL to GROUND. To protect against fire and electric shock hazards from the causes described in paragraphs 1. through 3. above, additional supplementary protection devices are employed. A fuse FUSE 20A and thermal cutoff TCO1 are inserted in the ungrounded current-carrying conductor PHASE ahead of the varistor surge suppression components. A second thermal cutoff TCO2 is placed in series with MOVs MOV2 and MOV3. The thermal cutoffs TCO1 and TCO2 are thermally activated non-resettable devices that permanently open the circuit upon reaching a specified temperature. MOV MOV1 is placed in close proximity to the thermal cutoff TCO1. MOVs MOV2 and MOV3 are placed in close proximity to thermal cutoff TCO2. The fuse FUSE 20A will open the circuit for failures of MOVs MOV1 or MOV2 resulting from the conditions described in paragraphs 1. and 2. above, thus providing a safe failure mode for these two MOVs for those causes of failure. The thermal cutoff TCO1 will open to provide a safe failure mode for MOV MOV1 if subjected to the limited current overvoltage described in 3. above. Thermal cutoff TCO2 will also provide a safe failure mode for MOVs MOV2 or MOV3 from a limited current overvoltage.
The prior art described above is effective in providing safe failure modes from the power frequency overvoltages applied from conductors PHASE-to-NEUTRAL and PHASE-to-GROUND, but it leaves some uncertainty with regard to safe failure when overvoltages described in 1. and 2. are applied to the ungrounded conductor NEUTRAL and the grounding conductor GROUND. This and other deficiencies and disadvantages of the prior art are detailed below:
(1) There is no fast acting fuse in the neutral conductor because UL and The National Electrical Code prohibit it. Consequently, safe failure of MOV MOV3 for power frequency overvoltages as described above in 1. and 2. depends upon heating thermal cutoff TCO1 to its operating temperature or rupturing the MOV body and blowing a soldered lead off the body to open the circuit. Heating thermal cutoff TCO1 to its operating temperature from the explosive failure mode of a high current overvoltage is unlikely and at best unreliable. A fast acting fuse in series with MOV MOV3 would avoid the blow-open scenario. However the use of such a fuse is undesirable because the fuse blowing could create a powerful inductive transient voltage surge that would be applied directly to the connected equipment. It would also leave the connected equipment unprotected from further surges and the power frequency overvoltage. (The fast acting fuse FUSE 20A in the conductor PHASE, being in series with the load, blows open to block the power frequency overvoltage as well as any inductive transient surge voltage that might be generated by the fuse blowing).
(2) After the fuse FUSE 20A or the thermal cutoff TCO1 opens, the surge suppressor device would be rendered inoperative and would have to be discarded.
(3) Heating of MOVs MOV2 or MOV3 from a limited current overvoltage would open thermal cutoff TCO2 and leave the connected equipment energized but unprotected from further surges and power frequency overvoltages occurring between conductors PHASE-to-GROUND and NEUTRAL-to-GROUND.
Other prior art is disclosed in Pat. No. 5,617,288 to Zaretsky, Apr. 1, 1997. It is intended to protect the MOV 14 from the Limited Current Overvoltage described in 3. above. This invention employs a circuit that scales and integrates the voltage applied to a MOV. The integrator output is intended to determine the length of time that the MOV is subjected to an overvoltage. This output is compared to a reference voltage that is intended to coincide with the time and magnitude of an overvoltage condition that is just below the MOV damage level. When the output reaches the reference level a relay is actuated that disconnects the MOV from the applied voltage.
This patent does not show any relationship of the voltage applied to the MOV and the voltage applied to the load. Thus it does not contemplate or teach protecting the equipment connected to the surge suppressor from the overvoltage.
This patent does not teach what would be required to protect from overvoltages in typical commercial devices that protect all three nodes--namely, phase-to-neutral, phase-to-ground and neutral-to-ground.
Overvoltages can occur during electrical distribution accidents any time after a complete power failure and before normal voltage is restored. A protection system should operate in such an event. There is no disclosure of how the system would operate if the device were not previously energized prior to the occurrence of an overvoltage. This is important because Zaretsky's patent uses active components that require a power source. Another disadvantage is that protection from high current overvoltages as described in 1. and 2. above is provided by a fuse in series with the MOV. This has the same disadvantages as described for the prior art surge suppressor in (2) above.
Other prior art is disclosed in Pat. No. 4,918,562 to Pulizzi, Apr. 17, 1990. A similarity exists in the teaching of this patent in the use of a capacitor 10 to store energy that is utilized to actuate a circuit breaker 112. The primary purpose of the Pulizzi system is to provide a means of disconnecting the source of AC power in the event of a complete power failure. The capacitor 10 is discharged through the circuit breaker actuator coil 142 upon the de-energization of relay K1. This assures system shut-down if a complete power failure occurs, and forces a manual orderly start-up when normal power is restored, for which the system is designed. The shutdown will also be activated if the power line voltage magnitude moves outside of a predetermined window defined as low and high voltage fault limits. This patent concerns damage to connected equipment or process failure involving connected equipment during power line voltage fluctuations such as "sags" and "swells". There is no mention of surge suppressor components, or speed of response required to protect such components or the connected equipment in the event of severe power line overvoltages as described in 1. and 2. above. No mention is made of the time required to sense the voltage and operate relay K1, which time delay will affect the speed with which the circuit breaker 112 will open the circuit. Because relay operation is slowest upon release, this system would not operate fast enough to protect metal-oxide-varistor surge suppressor components from a severe high current overvoltage.